Automatic test equipment for checking printed circuit boards has long involved use of a "bed-of-nails" test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail like, spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed-of-nails arrangement for contacting test points in a particular circuit board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This method typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture, superimposed on the array of test probes. During testing, the spring loaded test probes are brought into spring pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum may be applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. A lid on the housing is closed over the test fixture to form a vacuum seal between the lid and the probe plate. A vacuum applied to the region between the lid and the probe plate then moves the lid toward the probe plate which compresses the circuit board between the lid and the probe plate. This applies spring pressure from the probes to the board. Customized wire test fixtures of similar construction also can be made by using mechanical means, other than vacuum, to apply the spring force necessary for compressing the board into contact with the probes during testing.
One type of bare board circuit tester incorporating a wired test fixture is disclosed in U.S. patent application Ser. No. 07/824,854, filed Jan. 22, 1992, now U.S. Pat. No. 5,270,641 which is assigned to the assignee of this application. The circuit tester disclosed in the '854 application comprises a dual side access wired test fixture that permits the automatic testing of circuits on both sides of a printed circuit board simultaneously. The wired test fixture comprises upper and lower probe plates each having an array of test probes configured in a pattern corresponding to the patterns of test points in circuits on the top surface and bottom surface of the circuit board, respectively. The dual side access test fixture permits testing of circuits on the bottom side of the board by standard wired fixturing while testing of circuits on the top side of the board is provided by an external upper interface connector mounted over the upper probe plate. The upper interface connector allows the upper test probes to be electrically connected to the test electronics without having to route interface connections from the upper probe plate to the lower probe plate. Eliminating the need to transfer test signals from the upper probe plate to the lower probe plate provides for a larger test surface area on both the upper and lower probe plates since there is no need to occupy the probe plates with transfer pins. Another advantage is that test signals from the upper probe plate, being communicated directly from the upper interface connector to the external electric test analyzer, avoids use of additional wired interface pins.
As mentioned previously, the customized wired test fixtures are but one class of fixtures that may be used in a bare board circuit tester for transmitting test signals between the external electronic test analyzer and the printed circuit board; and more particularly, vacuum fixtures are but one category of wired test fixtures. A further class of test fixtures that may be used for bare board circuit testing is the so-called grid type fixture in which the test points on both sides of a board are contacted by flexible pins or tilt pins which can move or otherwise be positioned to contact the random pattern of test points on the board and transfer test signals from the board to sets of interface pins arranged in a grid pattern on the receiver. In these grid type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures because there is no need to individually hard wire the test probes to separate interface contacts for each differently configured circuit to be tested; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly.
In a grid type fixture, the wiring for the grid array remains constant, independent of the configuration of the particular circuit board. What does change, however, is what is referred to as the translator fixture. The translator fixture includes a bottom plate having a hole pattern corresponding to the grid pattern of openings in a standard pin grid array, and a top plate having a hole pattern corresponding to the random off-grid pattern of contact points to be tested on a printed circuit board. A number of electrically conductive translator pins (these can be flexible pins or tilt pins) are mounted in the holes of the top and bottom plates. As the translator pins travel through the translator fixture they are redirected by the hole patterns of the plates to provide individual conductive paths between the standard grid pattern and the off-grid pattern corresponding to the test points on the circuit board under test.
The construction of the grid type translator fixture is typically less labor intensive than the rewiring of test probes in a wired type test fixture. Therefore, it is often desirable to use a grid type test fixture when testing printed circuit boards having various different shapes and/or configurations. However, the downside of using a grid type test fixture to test circuit boards is that it requires an interface with a substantially more complex and costly external electronic test analyzer than that used with wired testing. Accordingly, the decision of whether or not to purchase a grid type circuit tester must take into account the competing factors of low labor costs for fixturing and the costly electronic test analyzer.
The present invention overcomes this dilemma by providing a bare board circuit tester that is capable of conducting either wired type testing or grid type testing for those occasions where one approach is more beneficial than the other. The tester of this invention is configured so that a grid type test fixture can be used interchangeably on the same test unit with a wired type test fixture. As a result the user may convert selectively from wired type circuit testing to grid type circuit testing when circuit boards having a variety of shapes and/or configurations are to be tested. The ability to switch from a wired fixture in such a situation allows the user to realize a substantial savings of time and labor associated with not having to individually hard wire the test probes for each different printed circuit board configuration.
The invention also permits the interchangeability of existing wired type test fixtures to grid type test fixtures without substantial modification, allowing the user to upgrade and increase the flexibility of existing wired circuit testers without having to incur the costs associated with having to purchase a dedicated grid type circuit tester. The invention also can be manufactured so that it permits grid type testing on both sides of a printed circuit board while avoiding the cost of using thousands of additional probes and switch points required in conventional grid testers.